Following the Crockett incident about 8 a.m. Wednesday, HISD advised principals to watch out for such envelopes, Cantu said. They were instructed to notify authorities, keep the envelope isolated in one room and turn off ventilation to that room, she said.

Shauna Dunlap, spokeswoman for the FBI in Houston, said she couldn’t comment on whether a third incident in two days would alter the FBI’s approach to investigating the envelopes.

“As far as the investigation, we would not disclose any techniques or strategies used in an ongoing investigation,” Dunlap said Thursday by email.

In the Dallas area, eight such incidents were reported on Tuesday and Wednesday, according to the Dallas Morning News.

In autumn 2010, the FBI investigated 17 reports of envelopes containing white powder at HISD campuses and more than a dozen such incidents at Dallas churches, mosques, businesses and schools. The envelopes contained a short message with a cryptic reference to al-Qaeda, officials said.